This invention relates to a film image reading out device, which reads out images on a film by means of laser light, and in particular to a film image reading out device provided with an image signal detecting system, which is suitable for reading out X-ray film images.
There are known various devices for reading out images recorded on an object by scanning linearly the object with a laser light beam and detecting transmitted light, reflected light, etc. coming from the object. Among them the device disclosed in JP-A-55-86970, which is an example, in which fluorescence of the object is measured, excites a fluorescent plate, on which X-ray images are recorded, by scanning it with a laser light beam and detects fluorescence generated thereby in order to read out the recorded images.
On the other hand, in a device reading out images recorded in the form of variations in the density on an X-ray film, etc., the images are read out by scanning the film with a laser light beam and by detecting the amount of the light transmitted by the film. In such a film image reading out device, variations in the optical density of the film constitute image information. However, when the laser light passes through the film, it is diffused. The degree of this diffusion depends on the optical density of the film and if the optical density is 0, the light is almost not diffused. On the contrary, the higher the optical density is, more strongly it is diffused. On the other hand, since there is a limit in the entrance angle of a photodetector, the optical density of the film based on the amount of light measured by the photodetector is different from that obtained by detecting all the diffused light (hereinbelow called diffused light density). That is, the dynamic range of the detected transmitted light is wider than the intrinsic dynamic range of the image on the film.
For this reason, heretofore such a kind of devices requires a photodetector system having a great S/N ratio and a wide dynamic range and moreover an A/D converter having a great number of bits, which digitizes signals. In general there exists no high speed A/D converter having a great number of bits, which was a reason, why it was impossible to obtain a high image reading out processing speed.